A target of ^7 Li is bombarded with a proton beam of current 10^ -4...

A target of ${ }^7 \mathrm{Li}$ is bombarded with a proton beam of current $10^{-4}$ ampere for 1 hour to produce ${ }^7$ Be of activity $1.8 \times 10^8$ disintegrations per second. Assuming that bombarding of 1000 protons produces one ${ }^7 \mathrm{Be}$ radioactive nucleus, the half-life of ${ }^7 \mathrm{Be}$ is estimated to be approximately